1. Field of the Invention
The present invention relates to a transmission gate, and more specifically relates to an improvement to a transmission gate which makes use of a substrate modulation for reducing distortion of transmitting signals.
2. Description of Related Art
A transmission gate is one of so called analogue switches and renders conductive or non-conductive between an input signal line and an output signal line in response to a control signal. An ideal transmission gate transmits an input signal to the output side without distortion when it is in a conductive state and outputs the input signal as it is as the output signal. However, when a transmission gate is constituted by a single element of a transistor, the resistance value of the transmission gate varies non-linearly depending upon the voltage of an input signal to thereby extremely distort the signal transmitted.
As one of measures for reducing such transmission distortion the resistance value of the transmission gate during its conductive state is linearized. As a specific example of such measure transmission gates of a CMOS circuit constitution as shown in FIG. 7 and FIG. 8 are used wherein a P channel MOS transistor (hereinafter simply called as PMOS transistor) and an N channel MOS transistor (hereinafter simply called as NMOS transistor) are connected in parallel.
FIG. 7 shows a fundamental constitution of a conventional transmission gate, wherein 1 is an NMOS transistor and 2 is a PMOS transistor. These transistors connected in parallel perform a switching function. When there are neither a control signal C nor its inverted signal *C, the both transistors 1 and 2 are in an "OFF" (non-conductive) state and the input signal line (or input terminal) and output signal line (or output terminal) are disconnected- When the gate receives a control signal C and its inverted signal *C, the both transistors 1 and 2 are rendered to an "ON" (conductive) state. At this moment, the NMOS transistor 1 and the PMOS transistor 2 exhibit resistance values of opposing characteristic with respect to the voltage of an input signal A, thereby linearity effected by the composite resistance value of these transistors is improved in comparison with that by the single element of a transistor. Further, pulse signals such as usual clock signals are used for the control signal, and its inverted signal is obtained by supplying the control signal via an inverter as illustrated in the drawing.
FIG. 8 shows a constitution of a conventional transmission gate which is subjected to a substrate modulation. In this circuit, a switching circuit 4 which connects the back gate of the transistor 2 to a power source Vcc (+) when there are neither a control signal C nor its inverted signal *C and a switching circuit 3 which connects the back gate of the transistor 2 to the input signal line when both a control signal C and its inverted signal *C are received, are added to the fundamental constitution as shown in FIG. 7. Via the function of these switching circuits 3 and 4 the back gate of the transistor 2 is subjected to a modulation by the input signal A, thereby the linearity of the resistance value of the transmission gate is further improved.
Via the application of the modular ion to the substrate as explained, the back gate of the transistor is applied a modulation by the input signal thereby the transmission distortion in the transmission gate is improved to some extent. However, the substrate modulation as the name indicates does not directly apply modulation to the back gate but applies to the substrate directly connected to the back gate to thereby apply a modulation to the back gate. For this reason the modulation is only applicable to either one of the transistors connected in parallel.
Namely, when the transmission gate is formed on a P type substrate, the NMOS transistor in a CMOS is formed in a P well region and is electrically isolated from the P type substrate so that the substrate modulation is only possible to the NMOS transistor. Further, when the transmission gate is formed on an N type substrate, in the same manner the PMOS transistor is electrically isolated from the N type substrate, thereby the substrate medulation is only possible to the PMOS transistor.
Accordingly, the resistance value of the transistor which can not be modulated by the input signal is not improved and the linearity of the composite resistance value when the parallel transistors are in a conductive state is still poor. As a result, with the above explained conventional transmission gate, the transmission distortion of signals is not sufficiently suppressed.